Backfiring through carbs and exhaust

[waterboymvp]

I recently did a top end rebuild on my 97 GTX (787) becuase the oil pump failed or was clogged so I blocked it off to go premix. Before I installed the rebuilt engine I drained all the fuel out because it sat all winter untreated. I put new mixed gas in (40:1) and installed engine. When I try to start it up it cranks fine but as soon as it starts to fire it will either cut out or backfire. Every once in while I could get to start and idle for 5 seconds or so and then it would cut off. I read a bunch of similer post and tried the following but had no luck.

1. Checked compression=140 psi both cylinders
2.New spark plugs
3. Clipped the ends of spark plug wires
4. Checked for spark
5.Checked rotary valve timing= perfect 147 open/65 close
6.Opened magneto housing= the whole at top of flywheel lines up straight up and down when mag cylinder is at top dead center. It was pretty clean in there, no envidence of the woodruff key shearing.
7. Changed rotary o-ring seal
8. Changed intake gaskets( thoght maybe an air leak)
9. Measured rotary valve clearance= within specs

I have not went through the carbs yet because it ran great until the oil pump failure. I have an identical GTX that runs perfect, so I'm going switch out the carbs to make sure its not a carb issue.

Is there anything else I'm missing? Anybody got any ideas?
Thanks in advance,,

 

[seadooya]

your rv setting, is it with the front/mag hole exposed, so that means, having the front/mag piston, at TDC?

Remove the sparkplugs, and insert 2-long screwdrivers, then rotate pto, to see if, at any moment, the pistons,rods, move the drivers, the same way.

 

[waterboymvp]

RV setting, I followed the manual. 147 open at TDC measured from bottom of MAG opening and 65 closed measured from top of MAG opening. I put the screwdrivers in the cylinders and they never move in the same direction.

 

[seadooya]

by blocking off the oil, did you still leave oil going to the rv cavity? if not, you just stripped that gear.

 

[waterboymvp]

I did leave the supply and return hooked up to the engine.so rotary should be safe.

 

[waterboymvp]

I did leave the supply and return hooked up to the engine.so rotary should be safe.

The engine is out sitting on the bench now. I can pull the RV to inspect it if you think the gear could be stripped.

 

[seadooya]

far-fetched, but you double check the plumbing of the delivery/return fuel lines.

 

[Minnetonka4me]

Sounds like you did EVERYthing...please keep us posted as to what you find. Sounds like the RV being off time, but you sound like you did that right.

 

[IDoSeaDoo]

I'm having the same problem. I recently rebuilt my carbs and changed pop-off pressure springs. I made my own pop-off pressure tester and put in the springs that gave me the correct pop-off according to the manual. I think now I have to adjust my low speed setting on my carbs to compensate. Wish me luck!